Inhee Lee

Inhee Lee

Contact Information:

2435 EECS
1301 Beal Avenue
Ann Arbor, MI 48109-2122

Research Interest:

low-power circuit design for miniaturized sensor nodes including a adaptive battery supervisory circuit, an capacitive energy harvester w/ maximum power point tracking, an energy-efficient oscillator, and a PVT variation tolerant voltage reference.

Current Research:

A 635pW Battery Voltage Supervisory Circuit for Miniature Sensor Nodes 
A low power battery voltage supervisory circuit is proposed for micro-scale sensor systems that provides power-on reset, brown-out detection, and recovery detection to prevent malfunction and battery damage. Ultra-low power is achieved using a 57pA, fast stabilizing two-stage voltage reference and an 81pA leakage-based oscillator and clocked comparator. The supervisor was fabricated in 180nm CMOS and integrated with a complete 1 mm3 sensor system. It consumes 635pW at 3.6V supply voltage, which is an 850x reduction over the best prior work. 

A Ripple Voltage Sensing MPPT Circuit for Ultra-Low Power Microsystems 
A maximum power point tracking (MPPT) circuit is proposed for micro-scale sensor systems that measures ripple voltages in a switched capacitor energy harvester. Compared to conventional current mirror type MPPT circuits, this design incurs no voltage drop and does not require high bandwidth amplifiers. Using correlated double sampling, high accuracy is achieved with a power overhead of 5%, even at low harvested currents of 1.4uA based on measured results in 180nm CMOS. 

Low Power Battery Supervisory Circuit with Adaptive Battery Health Monitor 
A battery supervisory circuit (BSC) is proposed for wireless sensor nodes that automatically adapts to varying battery health, as reflected by its internal resistance (RBAT), and establishes a constant effective threshold voltage. Compared to a conventional fixed-threshold BSC, the new design avoids oscillation and widens the usable range of battery voltages, independent of RBAT. RBAT is measured by inducing a test current using decaps and measuring the resulting battery RC response time. When tested with a 2¥ìAh battery and 11¥ìA sensor system, the BSC reduces the required hysteresis from 656mV to 77mV, increasing the usable battery voltage range by 2.7x.